Extraction of selected trivalent lanthanide and actinide cations by bis(hexoxy-ethyl) phosphoric acid

J. Inorg. Nucl. Chem., 1965, Vol. 27, pp. 1683 to 1691. Pergamon Press Ltd. Printed in Northern Ireland

EXTRACTION OF SELECTED TRIVALENT LANTHANIDE AND ACTINIDE CATIONS BY BIS(HEXOXY-ETHYL) PHOSPHORIC ACID* D. F. PEPPARD,G. W. MASON and G. GIFFINt Argonne National Laboratory, Argonne, Illinois

(Received 12 November 1964; in revised form 14 January 1965) Abstract--The extraction of Eu(lII) and Am(IIl) from aqueous chloride and perchlorate media into benzene and cyclohexane solutions of (n-C6HIsOCHsCHsO)zPO(OH), HDHoEP, has been shown to be 2.5 power dependent upon the formality of HDHoEP in the organic phase and inversely third-power dependent upon the concentration of hydrogen ion in the aqueous phase. The extracted entity, in the benzene system, is postulated as M(Y)(HYz)z, where (HY)s represents the dimer of HDHoEP, shown to be dirneric in dry benzene in the concentration range considered. In dry cyclohexane, HDHoEP is "empirically" trimeric and tetrameric, respectively, at 0.02F and 0.2F concentrations. The extractant is tentatively considered to exist as a mixture of trimers and tetramers in the "wet" cyclohexane of the system investigated. No attempt to formulate the extracted entity in Eu(III) and Am(IN) extractions has been made. Under comparable conditions, both Eu(llI) and Am(liD are extracted by HDHoEP with distribution ratio, K, values approximately 500 times greater in cyclohexane than in benzene.

A WIDE variety of mono-acidic phosphorus-based extractants, (X)(Y)PO(OH), in a carrier diluent have been reported as extractants of metallic cations in systems employing an aqueous mineral acid solution as the opposing phase. Specifically, di 2-ethyl hexyl phosphoric acid, (CzHs.CeHx20)2PO(OH), HDEHP, in benzene and toluene diluents has been studied as an extractant for lanthanides (III) and actinides (III) ~1~and for Th(IV) ~2~ from aqueous chloride and perchlorate phases. The measured distribution ratio, K, under otherwise constant conditions showed a third-power dependency upon the extractant concentration, a dependency equal to the charge on the ion for M(III) but less than the charge on the ion for Th(IV). Since, in such a system, the extractant exists as a dimer, symbolized as (HY)2, the extracted Th-containing entity is presumed to contain the equivalent of two monoionized dimers and two ionized monomers, Th(HYz)~(Y)2, and the extracted M(III)-containing entity is pictured as M(HYz)3 embodying the equivalent of three monoionized dimers. In order to determine whether lanthanide (III) and actinide (III) cations might also be extracted with a partial "monomer" stoichiometry under conditions favourable to "monomer chelation," the present study involving bis(hexoxy ethyl) phosphoric acid was undertaken. The ionized monomer of this compound, through use of both the negative oxygen attached to the phosphorus atom and the ether oxygen of one * Based on work performed under the auspices of the U.S. Atomic Energy Commission. t Present address: Susquehanna University, Selinsgrove, Pa. "~ • D. F. PEPPARD,G. W. MASON,J. L. MAXERand W. J. DmSCOLL,J. Inorg. Nucl. Chem. 4, 334 (1957); ~ D. F. PEPPARO,G. W. MASONand S. W. MOLXr~,J. Inorg. Nucl. Chem. 5, 141 (1957); c D. F. PEPPARD,G. W. MASON,W. J. DSJSCOLL and R. J. StRONEN, J. Inorg. Nucl. Chem. 7, 276 (1958). q2~D. F. PEPPARO,G. W. MASONand S. MCCARTV,J. lnorg. Nucl. Chem. 13, 138 (1960). 1683

1684

D.F. l~vl,~,

G. W. MASON and G. G~rrrs

h e x o x y - e t h y l g r o u p m a y t h e o r e t i c a l l y f o r m a seven m e m b e r e d r i n g i n c l u d i n g the metallic cation. EXPERIMENTAL

Nomenclature In accordance with previous usagey ¢,s~ the extractant, bis(fl-n-bexoxy ethyl) phosphoric acid (n-C6HtsOCH~CHtO)~PO(OH), is symbolized as HDHoEP. The symbols H, D, HoE, and P respectively indicate: a theoretically ionizable hydrogen, the prefix di-, referring to the number of organic groups replacing acidic hydrogen of orthophosphoric acid, the hexoxy ethyl group and phosphorus in an ortho phosphate structure. The extractant is further symbolically generalized as a monomer as HY and as a dimer as (HY)~. The concentration unit employed is formality, F, defined as the number of formula weights of solute per litre of solution. ~ The distribution ratio, K, of a given nuclide is defined as the concentration of nuclide in the upper divided by the concentration of nuclide in the lower of two mutually-equilibrated sensiblyimmiscible liquid phases, the concentrations being considered directly proportional to the radioactive counting rates of samples prepared under standard conditions, from suitable aliquots of the phases. The diluent, in this study benzene or cyclohexane, is referred to as the "carrier" diluentta~ not an "inert" diluent. The justification for this usage is apparent from inspection of Table 1. TABLE I.--VALUES OF Ks FOR EU(III) AND Am(IIl) IN THE SYSTEMSHDHoEP (in benzene or cyclohexane) vs. 1.00F (HX + NaX) CALCULA~D FROM Figs 1 AND 2 AND THE EQUAVXOS:

x , = Ko~.. {EH+L'/F~;0',}. Diluent

Benzene Benzene Cyclohexane Cyclohexane

X-

CIO4ClCIOjCI-

Kscsu~/Ka,Am ~

Eu (111)

Am (III)

5.4 x 10 2.6 x 10 3.0 x 104 1.4 x 10'

1"7 x 10 8'1 9'1 × 10a 4.3 x 10~

3"2 3.2 3"3 3.3

K8 (in cyclohexane)/Ks (in benzene) = (5.4 4- 0.5) x 102 KB (where X- = CIO4-)/Ks (where X- = Cl-) = 2.1 - 0.2

Sources of material Carbide and Carbon Chemicals Co. was the source of beta hexoxy ethanol, n-C6HasOCsH4OH, marketed as n-hexyl cellosolve or ethylene glycol monohexyl ether. The benzene and cyclohcxane used in radio nuclide distribution studies were respectively obtained from Allied Chemical, General Chemical Division and from Phillips Petroleum Co. The beta-active nuclides, 40.2 hr 14°La (with parent 12.8-day l~°Ba), 285-day t"Ce, 2.6-year x47Pm and (13, 16)-year x6s.t"Eu were obtained from the Isotopes Division of Oak Ridge National Laboratory. Daughter Z4°La was separated from parent t~°Ba (and from contaminant ,0y daughter of parent '°Sr present in the t'°Ba sample), and t " C c was separated, as Co(IV), from contaminant t~Pm, by liquid-liquid extraction techniques previously described, clb~ Liquid-liquid extraction was also employed in purifying ~'TPm from residual a"Cc ~tb~and 9ty.t~°~ The other beta-active nuclides employed, with the exception of 16eHo, were prepared by neutron irradiation of rare earths of normal isotopic composition in the form of oxides obtained in 99.9 + Yo purity from Research Chemicals, Inc. In the preparation of Dy tracer, secondary neutron capture led to the formation of 82-hr leeDy which has a 27.2-hr ~6'Ho daughter. By means of differential counting, both Dy and Ho data were obtained in a single expcrirrmnt as described in the section entitled Determination of Distribution Ratios. ca)D. F. PEPPARD, G. W. M A S O N and R. J. SIRONEN, J. Inorg.NucL Chem. I0, 117 (1959). (,)M. RANDALL and L. E. Yotn~o, Elementary Physical Chemistry, p. 59, Randall, Berkeley (1942).

Extraction of selected trivalent, ianthanide and actinide cations

1685

Alpha-active 470 year 2'1Am was obtained from Argonne stocks. The Signer tubes used in the isopiestic molecular weight determination of HDHoEP were obtained from Scientific Glass Apparatus Co., Bloomfield, N.J.

Preparation and purification of his hexoxyethyl phosphoric acid, (C6H,OCsH,O)2PO(OH), HDHoEP A solution consisting of 292 g (2'0 moles) of C6HlsOC~H,OH, 174 g (2.2 moles) of pyridine and 500 ml of benzene is added from a dropping funnel over a period of approximately 2 hr to a stirred solution of 154 g (1"0 mole) of POCls in 800 ml of benzene. After cooling, the mixture (solid pyridine hydrochloride and benzene solution of products) is added cautiously in small portions to two litres of water. (A great deal of heat is liberated in this hydrolysis step.) After this mixture has cooled to room temperature, the benzene phase is separated from the aqueous phase and the latter discarded. The benzene phase is scrubbed with three 500-ml portions of water to remove pyridine hydrochloride, hydrochloric acid and ortho phosphoric acid. To the resultant benzene phase, containing both monoester and di ester and, in general, some tri ester and small quantities of unreacted alcohol, is added aqueous 1"0 M NaOH to the neutral point as indicated by litmus paper dipped in the aqueous phase. The aqueous phase, containing the sodium salts of both mono and di esters, is scrubbed with five 500-ml portions of benzene to remove neutral ester and alcohol. This aqueous phase, after it is made 0.5 M in free NaOH by addition of 6 M NaOH, is heated at approximately 100°C for 2 hr to destroy any pyrophosphates. The cooled solution is then contatted with a 1-1. portion of di-ethyl ether. The sodium salt of the di ester reports nearly quantitatively and that of the mono ester only in slight part to the ether phase. (The aqueous phase may be set aside for recovery of the mono ester.) The pregnant ether phase is then scrubbed with six 1-1. portions of 0.5 M NaOH to reduce the content of mono ester salt to an acceptably low level. This ether extract is transferred to a large beaker immersed in an ice bath. To the stirred extract is slowly added a 500-ml portion of 4 M HCI (pre-cooled to approximately 5°C). The aqueous phase is discarded; and the ether phase, containing the product as free acid, is scrubbed with two 500-ml portions of 1 M HCI to remove traces of NaCI. The ether is then allowed to evaporate spontaneously from a shallow container in a well-ventilated hood. The remaining liquid is transferred to a 2-1. round-bottom flask which is then attached to an evaporator assembly as previously described. ~s~ (In this assembly, the flask, held in a sloping position, is rotated through nearly 360 °, alternately clockwise and counter-clockwise, thus continually exposing a new liquid surface and preventing bumping while volatile components are pumped off.) The pumping is ultimately done at below 0.5 mm pressure. This step serves to remove water, HCI and residual ether. The final product is a somewhat viscous, essentially colourless, liquid. Calculated for (CsH,OCIH40)2PO(OH), C16HssO6P: C, 54.22; H, 9"95; P, 8.74, Eq. wt. (354.4). Found: C, 54-07; H, 10.03; P, 8.58%. Eq. wt. by titration (357'8). (The C, H and P analyses were performed by Schwarzkopf Microanalytical Laboratory, Woodside, N.Y.)

Determination of distribution ratios The distribution ratio, K, for a given nuclide was obtained radiometricafiy by allowing the nuclide to distribute, at equilibrium, between the two mutually equilibrated liquid phases and then determining the radioactivity, in terms of counts per minute, alpha or beta, associated with a given aliquot of each phase. Equilibration was accomplished by three minute manual shaking of the two phases in 5-ml stoppered cylinders. The technique of transferring the nuclide from an aqueous salt phase to an organic phase suitable for evaporation on a platinum disc and the other experimental details have been described previously. 's~ The procedure utilized in counting ~eSDyand "*Ho on the same plate has been described. TM In each of the Am and Eu studies, the two tracers were simultaneously present, alpha and beta counting being performed as described previously. ~8) All data were obtained at 22 '-._ 1°C. (5~ D. F. PEPPARD, J. R. FERRAROand G. W. MASON,J. Inorg. Nucl. Chem. 7, 231 (1958). (8) D. F. PEPPARD, G. W. MASONand I. HUCHER,.I. lnorg. Nuel. Chem. 18, 245 (1961). (7) G. W. MASON, S. MCCARTY and D. F. PEPPARD,J. Inorg. NucL Chem. 24, 967 (1962).

1686

D. F. PEPPARLI,G. W. MACONand G. GIFFIN

Determination of molecular complexity of HDHoEP

The molecular complexity, 7, of HDHoEP was determined isopiestically in benzene and in cyclohexane by the SIGNER(~Jmethod at 22-O & 0.2°C as reported previously for other acidic phosphorus-based extractants, co)using tris(2-ethyl hexyl) phosphate, (GH&H,,O),PO, as the standard and assuming it to be monomeric. The values of 7 were confirmed by cryoscopic measurements in both benzene and cyclohexane. RESULTS

AND

CONCLUSIONS

and cryoscopic studies, Table 2, it is concluded that in benzene HDHoEP is dimeric throughout the concentration range investigated and that in cyclohexane it is the equivalent of “trimeric” in the lower concentration range and “tetrameric” at the higher. (Perhaps in dry cyclohexane the extractant exists as a mixture of dimers, trimers, tetramers, etc.; but it is tentatively assumed to exist as a mixture of trimers and tetramers.) From the isopiestic

TABLE 2.-MOLECULAR COMPLEXITY, 7, OF HDHoEP IN BENZENE AND CYCLOHEXANE AS DETERMINED: (A)ISOPIESTICALLYAT 22.0 f 0’5”C, TRIS 2-ETHYL HEXYL PHOSPHATE, ASSUMED MONOMERIC,AS STANDARD; (B) CRYOSCOPICALLY

Diluent Benzene

Method Isopiestic Cryoscopic

Cyclohexane

Isopiestic Cryoscopic

Equi. F of HDHoEP

rl

0.0312 0.251 0.0347 0.0695 0.127 0.189 0.0328 O-420 0.0301 0.104 0.200

2.02 2.00 2.12 2.13 2.15 2.16 2.65 4.29 2.70 2.95 3.57

In a system in which benzene is the carrier diluent, the distribution ratio, K, for both Eu(II1) and Am(W) is seen to be inversely third-power dependent upon the concentration of hydrogen ion in the aqueous phase, at ,u = 1.0, for both chloride and perchlorate media, Fig. 1. In a similar system (involving a lower concentration of HDHoEP) in which cyclohexane is the diluent, K is again inversely third-power hydrogen ion dependent for both Eu(II1) and Am(II1) for both chloride and perchlorate media, at ,u = 1.0, Fig. 2. In Fig. 3, the dependency of K upon the concentration of HDHoEP expressed in formality units for both Eu(II1) and Am(II1) is represented as 25power throughout the range investigated for the benzene system and as 25power in the lower concentration range for the cyclohexane system. It is presumed that in the benzene system the extractant exists in the “wet” benzene (*) a R. SIGNER,Am. 478,246 (1930);b E. P. CLARK, Industr. Engng. Chem. Analyt. 13, 820 (1941). Is) J. R. FERRARO, G. W. MASON and D. F. PEPPARD,J.Inorg. Nucl. Chem. 22,285 (1961).

Extraction of selected trivalent, lanthanide and actinide cations

'

fo,c,o;

l

~,

I

3

EutA,C I--2

2


Aml.v,CI-

I

O~

i -I

I-

i

SLOPES =-3'0

-2

i

-21

t

I

-2

~

-I

I )-3 0

Log _F H÷ FIG. l.--Hydrogen ion dependency of the extraction of Eu(III) and Am(Ill) into 0.106F HDHoEP, benzene diluent, from a 1.00F (HX q- NaX) solution. X = CIO4-, CI-. 5---

,

o

' fo, c,o;

-7

4

~,CI4~

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!

3

I

(

i Am,F O'c I 0~1

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,

J

I--

O~

SLOPES=-3'O

i

_,I -2

l

I -I

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I i- 2

0

Log F H+

2.--Hydrogen ion dependency of the extraction of Eu(ill) and Am(Ill) into 0.0187F HDHoEP, cyclohexane diluent, from a 1.00F (HX q- NaX) solution. X = ClOt-, CI-. FIG.

16

1687

1688

PEPPARD,G. W. MASON and G. GtrHr~

D.F.

primarily as a dimer, represented as (HY)2, and that the extraction of M(III) may be represented as: MA+a + 2.5(HY)% @ M(Y)(HYz)2o + 3Hx + (1) the subscripts A and O referring, respectively, to mutually equilibrated aqueous and organic phases. The expression for the distribution ratio, K, is thus: K = k[(HY)~]~5/[H+IAZ l

it

'

I [0,

CYCLOHEXANE l,

(2)

' Eu (rrr)

t,, Am (rrt)

{ VO Eu (rrt, BENZENE

--

3

Am(m)

t> 63-

_2

~

0

°-

& o

o N

12

--

I

.E

-ta

= ~e -

o o =

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I -2 0

Log F HDHoEP

Fro. 3.--Extractant dependency of the extraction of Eu(llI) and Am(IIl) into HDHoEP, in benzene vs. 0.25F HCIO4 and in cyclohexene vs. 1.0F HCIO4.

Letting F E represent the concentration of "extractant," i.e. HDHoEP, in the organic phase expressed in formality units, Expression (2) may be transformed into: K = KsF~'5/tH+IAa

(3)

an expression applying empirically to both benzene and cyclohexane systems. (The value of Expression (3) lies in the fact that Ks values for a variety of systems may be calculated and compared even without data concerning the molecular complexity of the extractant.) The compilation of Ks values of Table 1 emphasizes the diluent effect. At/z = 1.0, CIO4- or CI-, the Ks value in cyclohexane is approximately 500 times as big as the corresponding one in benzene. The ratio of the respective Ks values for Eu(III) and Am(III) is 3.2-3-3. The ratio of approximately 2.1 between pertinent Ks values in C104- and C1- systems is consistent with the chloride complexity constants recently reported for Eu(III) and Am(III), respectively 0.9 4- 0.3 and 0.9 4- 0.2. (z°~ Two facts to be remembered in referring to Table I are that in the calculation of Ks the verity of ca0~ ° D. F. PEPPARD,G. W. MASON and I. HUCHr.R, d. Inorg. Nucl. Chem. 24, 881 (1962); ~ M. L. BA~SAL, S. K. PATm and H. D. SrtARMA,d. Inorg. NucL Chem. 26, 993 (1964).

Extraction of selected trivalent, lanthanide and actinide cations

1689

Expression 1 is assumed and the Ks values refer specifically to systems in which /z = 1.0 with Na + as the supporting cation. The variation of log K with the atomic number, Z, for tracer-level lanthanides (III) in a system of constant composition organic and aqueous phases is shown in Fig. 4, the K values for Am(Ill) being included for comparison. The curves for the cyclohexane and benzene systems are mutually similar in general shape and slope. 4,

I I I I ' I (A) CYCLOHEXANE DILUENT

'~

(B) BENZENE DILUENT

'

3

/

//

3

o

2

.2.

,

o K

u ¢

..3

OF

~

"v

- - I I ;

Am

I

-1

,

57

59

61

I , i 63 6,5 Z

,

i 67

,

i , l-z 69 7I

FIG. 4.--Variation of log K with Z for lanthanides (llI) in the systems: (A) 0-0187F HDHoEP (cyelohexane) vs. 1.0F HCIO4; (B) 0.0375FHDHoEP (benzene) vs. 0.25 F HCIO4. It is evident that neither system shows much promise in effecting the separation of adjacent lanthanides in the lower Z range. However, in the higher Z range, especially beyond 67, such separations appear feasible, the separation factor, fl, for Tm(69) with respect to Er(68) being approximately 3.6 in-the cyclohexane system represented in Fig. 4. DISCUSSION

In Expression 1, the extracted entity in benzene is represented as M(Y)(HYz)2. However, it might be represented in several other ways including MYa(HY)2 , MYa(HY)(HY), etc., the first representing three ionized monomers plus an un-ionized dimer, the second three ionized monomers plus two un-ionized monomers, etc. The formulation used in Expression 1 was chosen because of its simplicity and because it seems the simplest modification of M(HYz)a which appears to be the reasonable representation of the extracted entity in systems in which a third-power extractant dependency is involved, tlc,6) A comparison of stoichiometries for HDEHP extraction systems involving monomerizing and non-monomerizing diluents suggests that the 2-5-power extractant dependency results from the relative stabilization of a monomeric anion, Y-, as opposed to a dimeric anion, HYz-.

1690

D.F.

PEPPARD, G. W. MASON and G. Girl=IN

It has been demonstrated recently, (n) that in a system consisting of bis 2-ethyl hexyl phosphoric acid, (CgHs.CeHx20)~PO(OH), HDEHP, in n-decanol as carrier diluent vs aqueous HCI, Tm(IIl), Y(III) and Sc(III) are extracted with a third-power extractant dependency in the low concentration of extractant region and with a sixthpower extractant dependency in the high concentration of extractant region. Similar results, with the third-power extending into a higher concentration of extractant region were observed for Eu(III) and Am(III). m) 0 • • .H--O. (A)

/e~O__M." .0j ~

~P/sO''°M (B~) / ~ 0 j

(B2) ~ ,./,,,0 / P~\O - - M

(C)

..

H I

0

(D)

H I

O--C--C-H ~'Pf H I / ~ O _ _ M .,.(~__C6 HI3

FIG. 5.--Possible modes of attachment of ionized (X)(Y)PO(OH) to a charged M atom.

In n-decanol diluent, HDEHP is considered to be monomeric at the concentration employed. (The monomerization of HDEHP by methanol has been demonstrated.) (9) Therefore, these dependencies have been interpreted as indicating M(Y)a and M(HYz)3 as co-existing extracted entities, the K for the first entity being thirdpower and that for the second entity sixth-power extractant dependent, since the extractant is monomeric HY. The concentration range at which the sixth-power mechanism becomes dominant is dependent upon the relative Ks values for the two extracted entities, as referred to M +3 in the aqueous phase, m) In the HDEHP in n-decanol study the postulation of monomers in the extracted entity appears logical, since the extractant is monomeric in the diluent, perhaps existing as a solvate, capable of ionizing in a manner equivalent to: R - - O - - H . . . O ~ P(OEH)2OH ~ R - - O - - H . . . O ~ P(OEH)~O- + H + (4) The anion represented by Expression 4 could chelate, through utilization of the alcohol oxygen, to form a six-membered ring with the M atom, (C) of Fig. 5. However, incorporation, by chelation, of an un-ionized monomer of HDEHP in the extracted entity in benzene solution, in which HDEHP is dimeric, requires formation of a four-membered ring, utilizing the two equivalent oxygens of the un-ionized monomer (B1) of Fig. 5, unless a mole of HOH serves the purpose of ROH in Expression 4, (C) of Fig. 5. If the monomer is incorporated without chelation, then tt~J G'. W. MASON, S. LEWEY and D. F. PEPPARD,J. Inorg. Nucl. Chem. 26, 2271 (1964).

Extraction of selected trivalent, lanthanide and actinide cations

1691

P--~ O is exposed, (B2) of Fig. 5. Such exposition of P ~ O without some compensating gain is not to be expected, considering the demonstrated stability of dimers of HDEHPtS, 9) and of the structurally similar di n-butyl phosphoric acid, HDBP. ~1~ However, since the oxygen of the P ~ O should be far less basic in HDHoEP than in HDEHP (considering relative inductive effects) and since in addition chelation to form a seven-membered ring is possible with HDHoEP, the exposition of a P - ~ O is more favourable than for HDEHP, and the exposition is partially compensated for by the gain in stability acquired through chelation. Both (C) and (D) of Fig. 5 involve a possibility of severe steric hindrance due to the R(or C6Hla). However, the structure of (C) does not involve an exposed P - ~ O. Therefore, it is to be expected that fewer monomers of type (D) than of type (C) might be present in an extracted entity. For Eu(IIl) and Am(Ill) it is suggested that formulation of the extracted entity, in benzene, as containing two ionized dimers, as in (A) of Fig. 5, and one ionized monomer, as in (D) of Fig. 5, is consistent with the data and with logical assumptions concerning coordination tendencies. In this discussion, it must be remembered that the 2-5-power extractant dependency has been demonstrated for Eu(llI) and Am(Ill) only. The extractant dependency of K for other lanthanides (II1) and actinides (11I) is unknown. From Fig. 3, it is evident that Expression (3) is not applicable to the cyclohexane system in the range of extractant concentration appreciably greater than 0.08 F. In this concentration range the existence of an appreciable portion of HDHoEP as tetramers (or higher aggregates) is probably responsible for the lowered extractant dependency. Since the cyclohexane presumably contains a mixture of HY polymers, no simple equivalent of Expression 1 may be written. Formulation of an "extracted entity," for Eu(III) and Am(Ill), in the cylohexane system is not possible on the basis of information presently available. The large diluent effect presented in Table 1 is a convincing argument against use of the term "inert" diluent. Acknowledgement---The authors thank Mrs. CAROLANDREJASICHfor the molecular complexity data taken from an extended "molecular complexity" study of a variety of mono-acidic phosphorus extractants presently in progress.

~12)~ D. DYRSSEN,Acta Chem. Scand. II, 1771 0957); b D. DYRSSENand F. KRASOVEC,Acta Chem. Scand. 13, 561 0959); c D. DVRSSENand L. D. HAY, Acta Chem. Scand. 14, 1091 (1960); ,t D. DVRSSENand L. D. HAY, .4cta Chem. Scand. 14, ilO0 (1960).